An AMOLED display element is a self-luminous element based on OLED (organic light emitting diode), and OLED adopts organic semiconductor materials and luminescent materials to conduct carrier injection and recombination under driving of an electric field to emit light. An AMOLED display device has a prospect of being used more widely because of its high luminance, clear picture quality, slim thickness, and excellent display performance.
The AMOLED display device includes tens of thousands of pixels, and each pixel includes an OLED and a pixel circuit for driving the OLED. The pixel circuit comprises a switching TFT (thin film transistor), a capacitor, and a driving TFT. The switching TFT charges a voltage corresponding to a data signal to the capacitor, and the driving TFT adjusts a magnitude of current supplied to the OLED according to a voltage of the capacitor; an amount of light emitted by the OLED is proportional to the current, and thereby luminance of the OLED is adjusted.
However, due to manufacturing issues or the like, there are characteristic variations in threshold voltages Vth of the driving TFTs and mobility among respective pixels, which results in the magnitudes of the currents for driving the OLEDs in the respective pixels are different, and luminance variations appear among the respective pixels. This will cause uneven luminance of the display picture, and may reduce a lifespan of the AMOLED display panel or render an image residue.
In view of the above, it is known to employ a compensation circuit to compensate for Vth drifting. An external compensation is a commonly adopted mode. The external compensation is implemented by adopting a pixel circuit with a compensation function and a customized driving chip to cooperate with each other.
In this compensation process, a large amount of data are computed and accessed to, so a random access memory (e.g., DDR, Double Data Rate synchronous dynamic random access memory) is required for data processing; also, a large amount of intermediate process data is required to be stored for compensation at a next power-on, and thus a memory (e.g., a flash memory chip) for storing electrical compensation data is necessary, and the data needs to be continuously updated with the aging of the TFTs and the OLEDs.
Since the time for the data updating is relatively long, one issue to be faced is that power-off may happen halfway. If data in the memory is just erased from one block, or data updating is being performed but has not been completed, power-off occurring at this moment will cause a loss of compensation data of a part of the pixels, so that the part of the pixels cannot be compensated for at the time of a next power-on.
The conventional data update mode to avoid data loss at power-off is that, each time before data in one block is erased, data in this block is copied to a spare block, and then the block is erased and written for accommodating new data, and a flag bit is modified; if power-off occurs at this moment, the data lost during the previous power-off can be read from the backup block according to the flag bit during a next power-on. The issue with this mode is that the backup block needs to be erased and written every time data of one block is updated; if more than one thousand blocks of the memory are updated each time, the backup block needs to be erased and written more than one thousand times, which seriously reduces a lifespan of the memory, and becomes a bottleneck in the product lifespan. Meanwhile, since data of each block is required to be copied up (involving the erasing and writing of the backup block) before being updated, the updating time is doubled, and the operation on the flag bits is very complicated and requires frequently switching between a reading mode and a writing mode of the memory; since the reading and writing modes of a large amount of data are different from those of a small amount of data, low efficiency occurs in the reading and writing of data.